The present invention relates to yttrium oxide-aluminum oxide composite particles and a method for the preparation thereof. More particularly, the invention relates to novel fine composite particles each consisting of a core particle of aluminum oxide and a cladding layer of yttrium oxide on the core particle as well as to an efficient method for the preparation of such composite particles. The composite particles are useful as a base material in the preparation of a sintered ceramic body consisting of yttrium oxide and aluminum oxide and as an ingredient in the protective film over the glass surface of a fluorescent lamp.
Sintered bodies consisting of yttrium oxide, i.e. yttria, and aluminum oxide, i.e. alumina, are widely utilized in the ignition tubes for sodium lamps and metal halide lamps and as a base material in the preparation of YAG (yttrium aluminum garnet) crystals. Such a sintered body is prepared usually by subjecting a powder blend consisting of a yttrium oxide powder having an average particle diameter of 1 .mu.m and an aluminum oxide powder having an average particle diameter in the submicron range to a sintering treatment in vacuum or in an atmosphere of air. As an inherent problem in the powder blending process involved in this preparation, the particles of the respective oxides can never be in a very intimately and uniformly mixed condition so that the powder blend is more or less microscopically not uniform from portion to portion.
It is an established technology that the glass surface of various kinds of lamps such as fluorescent lamps is provided with a coating layer of alumina as a protective film which has an effect of improving the durability and luminescent efficiency of the lamp while it is pointed out as an idea that replacement of the alumina in this application with another metal oxide such as yttrium oxide could be more efficient in respect of the above mentioned improvements. Despite this idea, fine particles consisting of or containing yttrium oxide as one of the principal constituents and suitable for such an application is not available in the prior art.